Reports reaching here that corpulent Sports Editor George Solomon entered last Sunday's National Symphony Five-Mile Run in Washington are seen as confirmation that the American mania for running, not so long ago the trendy pastime of sophisticates, has made its way down to the lowest social strata.

Bowler-hatted Bristons are bemused by this. Running is something horses do here. Riders exercising their steeds on Rotten Row in spacious Hyde Park in central London easily outnumber the few joggers, most of whom turn out to be resident or visiting Americans anyway.

It is possible to walk the streets safely here without the danger one so vividly remembers in Washington of being run down by heavy-set figures tramping through the early morning gloom in thick layers of sweatsuits, their rasping gasps drowning out the birds' greeting of the dawn.

The animal-loving English would report to the Humane Society the many Washington joggers who insist on dragging their poor pet dogs along with them. Londoners know that the proper way to exercise a dog is by walking, slowly, to allow time for the animal's natural needs and the master's chats about the miserable weather with passersby.

This is not to suggest that the British are indolent, not all of them, anyway. One recalls the chap climbing Everest some years ago Roger Bannister's breaking of the four-minute mile barrier and the handful of upper-class outdoors types currently circumnavigating the globe over both poles on sea and land with the blessing (but not the personal participation) of Prince Charles.

Everyone knows the English passion for walking in the country. The beautiful pastoral landscape is criss-crossed with miles and miles of posted public footpaths through national forests and private farmyards, with access guaranteeed to all.

But this sort of exercise, too, is taken in properly civilized fashion. First, one never starts out too early. A certain amount of sleep is needed to recover from the country house party the night before.

Then, the pace of the walk itself is limited by the weight of the picnic basket. This is not some shabby soft-drink cooler containing a few beers and hastily, thrown together sandwiches. It is, rather, a carefully selected assemblage of pates, cold meats, various salads, fresh bread and pastries, plus thermoses of tea and bottles of the right wine. The provisions must be sufficient for late morning tea, luncheon and afternoon tea. On even the most remote public footpaths, one can find, sooner or later, a comfortable bench suitable for these repasts.

But the most popular form of English exercise, as a prominent government minister reminded a group of American reporters only recently over lunch with two wines and cigars and brandy, is elbow-bending at the local pub.

Actually, the corner pub here rates right alongside any YMCA or health spa in Washington as a veritable gymnasium of exercise that goes well beyond the upper body-building strain of hoisting pint after pint from opening hour to closing time.

The classic English pub is so cramped and ill-furnished ("quaint and lively atmosphere" in the tourist brochers) that when everyone descends on it during the lunch hour and right after work in the evening, it is necessary to stand in a swirling crowd while drinking and eating.

The crush often is so great that one is forced out the door, providing some fresh air exercise while trying to shove back in.Muscling one's way to the bar for another pint of warm, watery lager or another sandwich of thin bread and an even thinner slice of unidentifiable meat -- both designed to aid the pub-goer's fitness program -- is the most taxing exercise of all, not unlike the task facing a Redskin running back seeking passage through the opponent's defensive line.

Small wonder there is little energy left the next morning for jogging through the deisel taxi fumes in Piccadilly Circus. The British, who have been around a long time, know better.

Confidence in President Carter, for example, started to fall here only after he began running. It hit rock bottom when Britons watch on their television screens as the leader of the free world ran himself into dangerous exhaustion while racing uphill.

The only real sign of hope to be seen from here was the front page Washington Post report that most Washingtonians stayed in their beds one recent weekend morning while several thousands others ran 26 miles through the city's streets with the U.S. Marine Corps. One assumes the corpulent sports editor was among those still dozing that day.